1. Field of the Invention
The invention relates to delivery systems for chemical agents such as herbicides, pesticides, fungicides, growth regulators and the like, and in particular to a nozzle arrangement for delivering a controlled spray of thin oil/water emulsion in the form of very-small droplets of uniform size. The invention is especially suitable for delivering thin oil/water inverts that form by surface tension and visco-elastic effects into droplets having an oil phase enclosing a water phase droplet.
2. Prior Art
In applying herbicides, pesticides, fungicides, plant growth regulators and similar agents to an area to be treated, it is highly desirable for a number of reasons to apply only the minimum amount of the agent necessary to achieve the desired effect, and to apply the agent accurately, i.e., only to the area being treated. Insofar as the agent is applied in an unnecessarily high concentration, or in a form that is not readily absorbed, or drifts away from the target site in the air, or evaporates before it is absorbed, the agent is not only wasted, but is a form of pollution.
Oil soluble and water soluble carriers for active agents are known. For the most part, the active ingredients used in the oil soluble and water soluble formulations are the same. Esters are examples of oil soluble carriers and amines are examples of water soluble ones. Esters are volatile. Even after reaching the vegetation or the like to be treated, the ester can volatilize before the active ingredient is absorbed. Similarly, amines are subject to evaporation. As an amine is delivered, a proportion of the water evaporates and is lost into the air. Both water soluble and oil soluble products should be delivered in a form that will minimize the loss of active material and the escape of active material into the environment, before absorption.
Both the form of the liquid being applied and the mechanics of application of the liquid are important considerations in connection with agricultural spraying and the like, for achieving the effects desired. In so-called "invert" formulations, an emulsion of water and oil is provided. Apart from the active ingredient(s), an emulsion comprises oil, water and an emulsifier or surfactant. The emulsifier is partly soluble in the oil phase and partly in the water phase, according to a hydrophile/lipophile balance factor that varies with temperature. By agitating a mixture of oil, water and emulsifier/surfactant, the immiscible oil and water are dispersed in the composition, with the emulsifier occupying surface boundaries between the oil and the water. Over time, the oil and water parts of an emulsion tend to separate, and may require agitation to reconstitute the emulsion.
Agitation affects the viscosity of the emulsion. Invert emulsions for application of agricultural agents are typically made viscous as a means to control droplet size. The viscosity is controllable by choice of the viscosity of the oil and/or by agitation to thicken the liquid. A more viscous (thicker) liquid formulation forms larger droplets when sprayed, than a less viscous liquid. Large droplets are more affected by gravity than by cross currents in the air, and are apt to fall directly onto the site rather than to drift in the wind.
U.S. Pat. No. 3,197,299--Stull discloses an example of a method and apparatus for spraying an invert emulsion. The invert is made as thick as mayonnaise, in an effort to form very large droplets or globs of material when sprayed via a nozzle device that impels a stream of oil and a stream of water together at an outlet to obtain agitation. The present invention takes a different approach to the problem of applying the active agent. Given that the same amount of material is dispensed, smaller droplets have been found to achieve more uniform coverage of foliage and the like than do larger droplets, provided the droplets can be kept small and uniform. Accordingly, the emulsion is made very thin, and is applied using a nozzle arrangement that forms droplets of uniform small diameter.
In a cross wind, large droplets of a thick invert fall more directly than small droplets. However, in practice, large droplets cannot be formed to uniform size, and as the droplets are formed and emitted from the sprayer, they separate into an aggregation of larger and smaller droplets. The smaller droplets of an aggregation of large and small droplets are subject to evaporation and drift, leading to widening of the swath of application from the sprayer. A spray of uniform small droplets is subject to displacement in a cross wind, but the displacement of the droplets is uniform because the droplets are uniform.
With a given quantity of agricultural chemical, smaller droplet application is more even on the smaller scale of the plant foliage. Unless the material is applied so heavily as to completely wet the leaves, large droplets spot the leaves with local concentrations of the active ingredient and relatively large spaces between them. Smaller droplets result in a larger number of smaller droplets, separated by smaller spaces. Thus the application is more even, and more effective.
Several parameters affect droplet size, including the viscosity of the liquid, the size and flow characteristics through the dispensing orifice and the like. When an emulsion of oil and water is emitted through a dispensing orifice, typically as a stream, surface tension progressively divides the stream along its length, as the stream flows from the point of emission. The oil in an invert emulsion forms a film on the water in a droplet and tends to hold the water in place. By balancing the viscosity of the liquid and the size of the stream emitted through the dispensing orifice, a balance can be struck. Dispensing orifices can be reduced in size to form smaller droplets. However, relatively smaller orifices increase the back pressure, and it is difficult or impossible to force viscous liquids through very small passages.
The effect of surface tension on an emitted stream of liquid is a much studied phenomenon. The inertia of liquid flowing through an orifice in a stream at first carries the liquid from the orifice in a solid stream having a uniform cross sectional diameter substantially equal to the internal diameter of the orifice. This lasts only for a short distance from the orifice. Surface tension acting on the stream causes the liquid to accumulate in droplets which are spaced by a distance related to the viscosity and surface tension of the liquid. Between adjacent forming droplets, a web of liquid is stretched and finally broken as the liquid is drawn into one of the two adjacent droplets. The emitted stream thus changes from a cylinder of liquid to a succession of droplets, with most of the liquid being drawn into a droplet adjacent its location. Near the midpoint between adjacent droplets a smaller droplet known as a satellite is often formed from a portion of the liquid which occupied the web that was stretched and broken as the stream formed into droplets. The satellite is about a tenth the size of the adjacent droplets.
In a thick invert formulation, the viscosity of the liquid makes it difficult or impossible to discharge the liquid through small orifices, to form small droplets. The thick invert liquid therefore is discharged through relatively larger orifices. When a stream of the liquid is emitted, droplets form in a wide range of sizes, both small and large. Whereas the basic objective of using a thick invert is to achieve large droplet sizes for better drift control, this objective is only partly met.
Provided the liquid can be forced through the dispensing orifices, it is generally possible to achieve a small droplet size by using small diameter orifices. According to the present invention, droplets are formed using a thin invert emulsion emitted through capillary sized passages, i.e., having a diameter which is small enough that surface tension causes the liquid to fill the internal diameter of the passages. Drift is controlled by the mechanics of application of the product, such as dispensing in the immediate area of the target, rather than by relying on large droplet size. The better coverage of uniform small droplets provides high efficacy and substantially reduces the volume of the agent needed to achieve the desired effect, per unit of coverage area.
Down to a certain diameter, a small conduit can be arranged by drilling a bore in a nozzle wall or by placing a tube through a nozzle wall, for example as in U.S. Pat. No. 5,110,048--Waldrum. At some point, however, reducing the conduit size causes manufacturing problems as well as problems in use. It is impractical, for example, to attempt to form an orifice having an internal diameter less than about 0.015 inch. Such a hole is too small to drill dependably to a uniform size. Even when formed, the orifice is likely to become plugged by solid material in the liquid to be dispensed. Once plugged, the orifice is almost impossible to clean.
The present invention is intended to provide a very small orifice, preferably on the order of 0.002 to 0.015 inch or smaller. The orifice preferably is used to form droplets of about 250 to 300 microns diameter, and is especially useful for application of thin invert emulsions. Moreover, according to one aspect of the invention, the orifices can be readily cleaned.
Assuming that a uniform application of small droplets can be accomplished, the amount of active material applied to a site, as well as the volume of carrier liquid can be reduced. With uniform and accurate coverage, a more concentrated agent can be applied. Smaller, lighter equipment can be used to apply the material. The thin invert formulation, characterized by an oil phase on a water phase, reduces problems with evaporation. The oil also assists in penetration of the waxy surface of vegetation. In short, the overall effectiveness of application is improved.
According to the invention, a dispensing nozzle is provided with uniform, very-small diameter orifices by slotting at least one of two abutting faces between relatively movable parts defining a nozzle wall. The orifices can be formed by cutting with a broach to form an array of slots in a conical structure adapted to fit a complementary conical seat. Micro-orifices are defined between the conical structure and seat. For cleaning, the conical structure can be lifted from the seat, thus flushing the slots. A filtration arrangement including a screen reduces the tendency of the nozzle to clog. The dispensing nozzle provides an efficient and cost effective solution to the problem of forming uniform small droplets, for use with thin invert emulsions.